Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes

Hydrogen-Bond and Solvent Dynamics in Transition Metal Complexes: A Combined Simulation and NMR-Investigation

Self-assembling ligands through complementary hydrogen-bonding in the coordination sphere of a transition metal provides catalysts with unique properties for carbon–carbon and carbon–heteroatom formation. Their most distinguishing chemical bonding pattern is a double-hydrogen-bonded motif, which determines much of the chemical functionality. Here, we discuss the possibility of double proton transfer (DPT) along this motif using computational and experimental methods. The infrared and NMR spectral signatures for the double-hydrogen-bonded motif are analyzed. Atomistic simulations and experiments suggest that the dynamics of the catalyst is surprisingly complex and displays at least three different dynamical regimes which can be distinguished with NMR spectroscopy and analyzed from computation. The two hydrogen bonds are kept intact and in rapid tautomeric exchange down to 125 K, which provides an estimate of 5 kcal/mol for the barrier for DPT. This is confirmed by the simulations which predict 5.8 kcal/mol for double proton transfer. A mechanistic interpretation is provided and the distribution of the solvent shell surrounding the catalyst is characterized from extensive simulations

Water-Soluble Co(III) Complexes of Substituted Phenanthrolines with Cell Selective Anticancer Activity

Transition metal complexes with substituted phenanthrolines as ligands represent potential anticancer products without the drawbacks of platinum complexes that are currently marketed. Here, we report the synthesis and cell selective anticancer activity of five new water-soluble Co­(III) complexes with methyl substituted phenanthroline ligands. The complexes were characterized by elemental analysis, NMR, FAB-mass spectrometry, FTIR, electronic spectroscopy, and single crystal X-ray diffraction. Possible interaction of these complexes with DNA was assessed by a combination of circular dichroism, UV–vis spectroscopy titration, and ethidium bromide displacement assay, and the results indicated that DNA interaction is weak for these complexes. Cellular uptake and cytotoxicity of complexes at low concentrations were assessed by flow cytometry on PC-3 cells, while their effect on intracellular mitochondrial function was measured by MTS assay on HeLa and PC-3 cell lines. These complexes showed selective cytotoxicity with a significantly higher effect on intracellular mitochondrial function in PC-3 cells than in HeLa cells. At low concentrations, complex <b>2</b> had the highest cytotoxic effect on PC-3 cells, inducing around 38% cell death, and the correlation of cytotoxicity of these complexes to their hydrophobicity indicates that an appropriate value of the hydrophobicity is essential for high antitumor activity

Deltoid versus Rhomboid: Controlling the Shape of Bis-ferrocene Macrocycles by the Bulkiness of the Substituents

Precise structural control of heteroannularly disubstituted ferrocene (Fc) structures is very challenging as the high rotational mobility of the Fc unit allows a large conformational diversity. Herein we present the syntheses, characterization, and electrochemical investigation of two complementary bis-ferrocene macrocycles, built up via Sonogashira cross coupling and intramolecular ring-closing reaction. While the X-ray structure of 1,2-ethynylbenzene bridged bis-ferrocene complex <b>1</b> shows a deltoidal conformation, a stretched oriented rhomboidal bis-ferrocene metallacycle <b>2</b> is formed when the peripheral benzene rings are decorated with bulky <i>tert</i>-butylsulfanyl groups. VT-NMR spectroscopy is used to assign the rotation of the embedded Fc units in rhomboid <b>2</b>. Moreover, cyclic voltammetry (CV) of deltoid <b>1</b> and rhomboid <b>2</b> indicate that electronic communication between both ferrocenyl groups can be neglected, while the electrostatic through space coupling is significant

Water-Soluble Co(III) Complexes of Substituted Phenanthrolines with Cell Selective Anticancer Activity

Transition metal complexes with substituted phenanthrolines as ligands represent potential anticancer products without the drawbacks of platinum complexes that are currently marketed. Here, we report the synthesis and cell selective anticancer activity of five new water-soluble Co­(III) complexes with methyl substituted phenanthroline ligands. The complexes were characterized by elemental analysis, NMR, FAB-mass spectrometry, FTIR, electronic spectroscopy, and single crystal X-ray diffraction. Possible interaction of these complexes with DNA was assessed by a combination of circular dichroism, UV–vis spectroscopy titration, and ethidium bromide displacement assay, and the results indicated that DNA interaction is weak for these complexes. Cellular uptake and cytotoxicity of complexes at low concentrations were assessed by flow cytometry on PC-3 cells, while their effect on intracellular mitochondrial function was measured by MTS assay on HeLa and PC-3 cell lines. These complexes showed selective cytotoxicity with a significantly higher effect on intracellular mitochondrial function in PC-3 cells than in HeLa cells. At low concentrations, complex <b>2</b> had the highest cytotoxic effect on PC-3 cells, inducing around 38% cell death, and the correlation of cytotoxicity of these complexes to their hydrophobicity indicates that an appropriate value of the hydrophobicity is essential for high antitumor activity